In the preparation of kraft cellulose pulp, particularly by continuous cooking techniques, wood chips (or like comminuted cellulosic fibrous material) are normally presteamed to remove the air from the chips and to preheat them to a temperature of about 100-120.degree. C. According to Rydholm "Pulp Processes", a higher temperature leads to both carbohydrate hydrolysis and liquor condensation. After this the chips typically pass into a chip chute associated with a high pressure feeder, in which they are entrained in cooking liquor at a temperature of about 115-130.degree. C., and while being transferred and either at the top of the digester or in a separate impregnation vessel, are impregnated with the cooking liquor at this temperature, typically for about 15-30 minutes. Afterwards the slurry is heated to a temperature of about 150-170.degree. C. for actual cooking, which takes place in the digester. There are about 400 continuous digesters operating worldwide in this manner, which has long been accepted as the appropriate way of preparing chips for cooking, and then effecting continuous cooking.
According to the present invention it has been found that this conventional, widely accepted, method of handling chips prior to kraft cooking may in fact have an adverse effect on the strength of the pulp produced especially when producing pulp with lower than normal Kappa No. The conventional wisdom in the art has failed to take cognizance of the fact that when the chips are at the temperature that they are during conventional impregnation that the acid substances that are formed, including by hydrolysis reactions and during delignification, often provide a sufficient build up of acid--prior to neutralization with the alkali liquid in which the chips are impregnated--that the acid can attack the cellulose in the chips, and/or cause lignin condensation, both of which are adverse to the subsequent pulping operations, and at least the acid attacking the cellulose having an adverse affect on pulp strength. According to the present invention it has been recognized that the diffusion of the alkali into the wood chips is not quick enough to neutralize the acidic substances, and that therefore a different approach must be taken if maximum ultimate pulp strength is to be obtained from a given source of raw cellulose material.
According to the present invention, instead of heating the chips to the temperature range of 110-130.degree. C., in which the hydrolysis, delignification, and other chemical reactions start (and produce acid), the chips are instead "cold" impregnated. That is they are subjected to impregnation in such a way that any chemical reactions producing acid are slow enough so that diffusion of the alkali from the impregnating liquor is sufficiently rapid so as to neutralize the acid formed before it can attack the cellulose and/or cause lignin condensation. If no chemical reactions take place, there is sufficient time for the alkali to diffuse into the cells so that when acid is produced alkali is immediately available to neutralize it. It has been surprisingly found that according to the present invention that the alkali concentration can be relatively low while these results are satisfactorily achieved, and that ultimately a pulp with improved strength compared to pulp produced by conventional impregnation techniques is provided.
According to one aspect of the present invention a method of impregnating comminuted cellulosic fibrous material (such as softwood chips, but also including a wide variety of other cellulose materials such as hardwood, bagasse, and agricultural materials) comprises the steps sequentially continuously: (a) Removing air from the comminuted cellulosic fibrous material and preheating the material to a temperature above ambient. (b) Soaking the material in an alkaline liquid (preferably containing sulfide) at a temperature such that if acid-forming chemical reactions take place at all, the reactions take place slowly enough so that the acid produced will be neutralized by the alkaline liquid before the acid can damage the cellulosic material or cause lignin condensation; and wherein the alkali concentration of the liquid is sufficient to neutralize any acid produced; and for a time period sufficient to substantially completely impregnate the cellulose material with alkali. And, (c) raising the alkali-impregnated cellulosic material to cooking temperature to effect continuous cooking thereof to produce cellulose pulp. The is cooking is preferably to a lower than normal Kappa No., i.e. for kraft cooking lower than Kappa No. 25, even lower than Kappa No. 20.
In the practice of the method as described above, step (a) is typically practiced by presteaming in a conventional manner, except that the maximum temperature at which the chips are steamed is about 110.degree. C. Preferably presteaming to preheat the chips and remove the air takes place at about 80-110.degree. C. (more desirably 90-105.degree. C., and perhaps most desirably at about 90-100.degree. C.) for a time period of about 5-60 minutes (typically about 10-30 minutes). It is advantageous (but not necessary) to have alkali present during steaming. This way condensates formed during steaming become alkaline. Alkali can be added to the chips before or during steaming in the form of ash or powder, white liquor, black liquor, or the like. Step (c) is preferably practiced in a continuous digester, with a cooking temperature of about 145-180.degree. C.
Step (b), which is the essence of the present invention, for most cellulosic fibrous materials is practiced at a temperature of about 80-110.degree. C., more desirably about 90-105.degree. C., or alternatively 80-100.degree. C. Treatment time is typically at least half an hour, and the treatment time may be substantially indefinite as long as the pulp mill can afford this aspect of the processing to proceed very slowly, and as long as there is no adverse affect on the cellulosic material (which there typically would not be in a relatively mildly alkaline liquid). As a practical matter the time period for treatment would rarely be over 72 hours, and more desirably is about 1-4 hours with about 2-3 hours being optimum. The soaking need not be at high pressure; in fact the only reason for utilizing superatmospheric pressure at all would be to compress possible gas bubbles to make it easier for the chips to sink. In conventional impregnation the pressure is about 10-20 bar, whereas according to the present invention step (b) would be practiced at 0-15 bar (gauge), preferably either 0 to 5 bar or 1-5 bar.
Step (b) typically is practiced so as to dissolve at least about 8% of the wood material (preferably about 10-20%), and also to dissolve at least about 15% of the lignin, typically about 20-40%. Substantially all of, or at least a majority of, the dissolved lignin (and other organic solids, like hemi-cellulose) may be removed before step (c), which has advantages in the ultimate entire treatment sequence.
Between steps (b) and (c) there may be a further step of heating the cellulosic material to a temperature of about 110-150.degree. C. for about 10-90 minutes (preferably 120-140.degree. C. for about 10-30 minutes). During the practice of step (b) while higher alkali concentrations can be utilized, it is not necessary to use an alkali concentration of over about 1.0 mole of NaOH per liter (or the equivalent of other alkalis), and in fact an alkali concentration of about 0.75 mole of NaOH per liter or less is very effective.
According to another aspect of the present invention a method of impregnating wood chips is provided comprising the steps of sequentially: (a) Steaming the chips to remove air from them and to heat them to a temperature of about 80-100.degree. C. (b) Soaking the chips in an alkaline liquid (preferably also including sulfide) for about 1-72 hours at a temperature of about 80-110.degree. C. (preferably about 80-100.degree. C.) to impregnate the chips with the alkali and dissolve at least about 8% of the wood. And, (c) raising the wood chips to cooking temperature and effecting continuous cooking thereof to produce cellulose pulp.
The invention also relates to a novel kraft pulp, the pulp according to the present invention having improved strength compared to kraft pulps otherwise exactly produced only using conventional impregnation techniques as opposed to the cold soaking technique of the invention. That is according to another aspect of the present invention kraft paper pulp having improved strength is provided that is produced by the steps of sequentially continuously: (a) Removing air from the comminuted cellulosic fibrous material and preheating the material to a temperature above ambient. (b) Soaking the material in an alkaline liquid (preferably also containing sulfide) at a temperature such that if acid-forming chemical reactions take place at all, the reactions take place slowly enough so that the acid produced will be neutralized by the alkaline liquid before the acid can damage the cellulosic material or cause lignin condensation; and wherein the alkali concentration of the liquid is sufficient to neutralize any acid produced; and for a time period sufficient to substantially completely impregnate the cellulose material with alkali. And, (c) raising the alkali-impregnated cellulosic material to cooking temperature and effecting continuous kraft cooking of the cellulose material to produce chemical pulp having enhanced strength ompared to otherwise identically treated material only impregnated n a different manner than impregnation according to step (b). Preferably step (c) is practiced at about 150-180.degree. C. to produce kraft pulp in a continuous digester.
The invention also relates to a system for treating cellulosic fibrous material to produce chemical pulp. The system comprises the following components: Means for removing air from the comminuted cellulosic fibrous material and preheating the material to a temperature above ambient. A first vessel for soaking comininuted cellulosic material from the means for removing air therefrom, and need only withstand a pressure of about 5 bar or less, and an alkaline liquid at a temperature of less than about 110.degree. C. A continuous digester for cooking the cellulosic fibrous material from the first vessel. And, means for transferring the cellulosic material from the first vessel to the digester. The digester is preferably a continuous digester and the transferring means also pressurizes the material during transfer (e.g. may comprise a conventional high pressure feeder). At least one impregnation vessel may be provided between the first vessel and the digester, particularly in retrofit operations, or cold soaking may be done at the top (e.g. impregnation zone) of a single vessel hydraulic digester. Also the system may further comprise means for removing a first liquid from the first vessel, the first liquid containing dissolved lignin/solids, and replacing the first liquid in the first vessel with a second liquid (such as wash or bleach plant filtrates) having a lower content of dissolved lignin/solids than the first liquid. Utilizing the teachings of the invention it may even be possible to eliminate the conventional high pressure feeder in continuous kraft systems. For example, the air removing means may comprise a chip bin with steaming, and the chip bin and first vessel are above the transferring means, and the transferring means is above ground level and consists essentially of a pump [that is being devoid of a high pressure feeder].
It is the primary object of the present invention to simply yet effectively increase the strength of chemical pulp, particularly produced by continuous digesting. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.